A vehicle with hybrid propulsion comprises an internal combustion thermal engine, which transmits torque to the driving wheels by means of a transmission provided with a gearbox, and at least one electric machine, which is electrically connected to an electric accumulation system and mechanically connected to the driving wheels.
The following modes are possible while traveling: either a thermal operating mode, in which the torque is generated only by the thermal engine, and the electric machine may work as generator for recharging the accumulation system; or an electric operating mode, in which the thermal engine is off and the torque is generated only by the electric machine working as engine; or a combined operating mode, in which the torque is generated by both the thermal engine and by electric machine working as engine. Furthermore, in order to increase total energy efficiency during all steps of decelerating, the electric machine may be used as generator for a regenerative deceleration in which the kinetic energy possessed by the vehicle is partially converted into electricity, which is stored in the accumulation system, instead of being fully dissipated in friction in the brakes.
When the electric machine is disconnectable from the driving wheels, i.e. when the electric machine is mechanically connected to a primary gearbox shaft or is mechanically connected directly to a drive shaft of the thermal engine, the electric machine may be used to start the thermal engine itself, thus also performing the function of starter motor (as described, for example, in U.S. Pat. No. 5,337,848A1 and in patent application GB2335404A).
When the thermal engine is “cold”, the thermal engine requires the application of a very high torque (indicatively even 2-4 times higher than the starting torque needed to start a “warm” thermal engine) to the drive shaft, because the lubricant oil inside the thermal engine is not very fluid and thus opposes a high mechanical resistance due to the low temperature. A traditional starter motor can apply a very high torque to the drive shaft of the thermal engine by virtue of a very high gear-down ratio (e.g. even 1:10) between the starter motor and the drive shaft; instead, the electric machine of a vehicle with hybrid propulsion is normally mechanically connected to the drive shaft of the thermal engine with a low gear-down ratio (often unitary, i.e. 1:1, and thus without any gear-down). Consequently, the electric machine of a vehicle with hybrid propulsion may not be capable of generating the very high starting torque needed to start the thermal engine when the thermal engine is “cold”.
In order to solve the aforesaid problem, in patent application US2002117860A1 it is suggested to mechanically couple the electric machine to the drive shaft of the thermal engine with two different, automatically selectable transmission ratios: a first, more geared-down ratio is selected during a step of starting of the thermal engine, while for the remaining use of the electric machine a second, more direct ratio (i.e. less geared-down ratio) is selected. However, this solution is constructively complex because a servo controlled gearbox must be interposed between electric machine and drive shaft.
In order to solve the drawback described above, in patent applications W02007131838A1, US2005155803A1, U.S. Pat. No. 6,354,974A1 and US2009017988A1 the electric machine is separated from the drive shaft of the thermal engine by a clutch; in order to start the thermal engine, the clutch is opened, the electric machine is rotated with “no load” and the clutch is closed with the electric machine rotating so as to also exploit the inertia (i.e. the kinetic energy) possessed by the electric machine to rotate the thermal engine. However, such “launched” starting methods may not be sufficient to start a sporty, high-performance thermal engine, which has a high displacement and a high number of cylinders. Furthermore, the “launched” starting method allows to apply a high starting torque to the drive shaft needed to start a “cold” thermal engine, but is redundant to start a “warm” thermal engine; consequently, when the thermal engine is “warm”, the “launched” starting method may impose unnecessarily high mechanical stress on the transmission components and determine an unnecessary delay in starting the thermal engine related to the time needed to “idle” the electric machine.
It has been further suggested to start the thermal engine with the vehicle moving, i.e. when the driver requires forward vehicle motion the vehicle itself is initially moved only by the electric machine working as engine (thus solely in electric traction) and only later is the clutch which connects the drive shaft of the thermal engine to the electric machine and to the driving wheels closed to better exploit vehicle inertia to rotate, and thus start, the thermal engine.
By virtue of the vehicle inertia, the starting of the thermal engine is guaranteed in relatively rapid times; however, when the clutch which connects the drive shaft of the thermal engine to the driving wheels is closed, a reduction (or a cancellation or even an inversion) of the vehicle acceleration occurs due to the braking effect of the thermal engine; such a reduction of vehicle acceleration is very annoying because it is clearly perceived by the driver. It is worth noting that the driver does not intervene in any manner on the decision to start or stop the thermal engine: the only command decided by the user is to press the accelerator pedal and when the driver presses the accelerator pedal the vehicle is expected to accelerate uniformly and not to “suddenly” brake because an electronic control unit has autonomously decided to start the thermal engine. Furthermore, the braking effect of the thermal engine determines the onset of longitudinal oscillations, which are particularly annoying because they are clearly perceived and very uncomfortable.
Finally, during the starting of the thermal engine vibrations may be generated on the drive shaft, which are transmitted to the driving wheels determining further longitudinal oscillations of the vehicle. As previously mentioned, the longitudinal oscillations are particularly annoying because they are clearly perceived and very uncomfortable.